For many applications in the petroleum and automotive industries there is a need for elastomeric materials with good oil resistance at elevated temperatures. There is a particular need for materials that are flexible and soft (low in hardness) with good resistance to heat and compression set.
It is generally known in the art to employ curable polyacrylate elastomers to manufacture high performance rubber parts having excellent resistance to lubricating oils and greases which are therefore useful in selected automotive applications and the like. The gum rubber vulcanizates are either polyacrylate elastomers derived from copolymerization of acrylic acid ester monomers (e.g., ethyl, butyl, and methoxyethyl acrylate and can include some vinyl acetate), polyethylene/acrylate elastomer derived from copolymerization of ethylene monomer and acrylic acid ester monomers (e.g. ethylene and methyl acrylate and can include other comonomers and grafts, see for example U.S. Patent Publication No. 2002/0004568 A1 incorporated herein by reference), or polyperfluoroalkyl acrylate elastomer derived from polymerization of fluorinated acrylic ester monomer (e.g., 1,1 dihydroperfluoro-n-butyl acrylate). The polyacrylate elastomers also can be functionalized by incorporating a relatively small amount of an additional comonomer such as an acrylate glycidyl ester, maleic acid or other comonomer having a reactive group including acid, hydroxyl, epoxy, isocyanate, amine, oxazoline, chloroacetate or diene. These functionalized polyacrylate elastomers can then be cured using a curative co-agent containing functional groups that covalently bond to the functionalized reactive sites of the polyacrylate elastomer.
One problem associated with the prior art curable polyacrylate elastomers is the inherent rheological limitations of high viscosity and low melt flow of their cured or partially cured state. Consequently, physical blending followed by compression molding and subsequent curing is usually necessary to achieve acceptable properties rather than extrusion or injection molding directly to a finished part (as discussed above). However, in European Patent 0 337 976 B1 and in U.S. Pat. No. 4,981,908 thermoplastic elastomer compositions are disclosed comprising blends of polyester resin (including segmented polyether ester elastomers commercially available under the trademark HYTREL® (E. I. du Pont de Nemours and Company, Wilmington, Del. (“DuPont”)) and dynamically vulcanized, covalently cross-linked acrylate rubber (including ethylene/methyl acrylate terpolymer containing about one mole percent of a carboxylic acid containing comonomer, commercially available under the trademark VAMAC®. (DuPont). The covalent cross-linking in these disclosures is achieved by employing a functionalized polyacrylate elastomer in combination with reactive difunctional cross-linking agent. However, almost all of these difunctional cross-linking agents can also react with the ester units in the polyalkylene phthalates (i.e., an amine, hydroxyl or carboxylic acid group will exchange with the ester groups and epoxy or acid groups will add to the hydroxyl end groups), which leads to high viscosity and lack of reproducibility.
In U.S. Patent Application Publication No. 2004/0115450 there is disclosed a curable thermoplastic elastomeric blend comprising a polyalkylene phthalate polyester polymer or copolymer and a crosslinkable poly(meth)acrylate or ethylene/(meth)acrylate copolymer vulcanizate rubber in combination with a peroxide free-radical initiator and an organic multiolefinic coagent to crosslink the rubber during extrusion or injection molding of the blend. It is taught there that the polyester hard segment blocks in the copolymer should have high melt temperatures to obtain useful elastomeric blends for high temperature service. However, it is generally found that hard segment high melt temperatures increase polymer hardness and reduce flexibility.
It is an objective of the present invention to provide softer and more flexible thermoplastic elastomeric blends which provide excellent resistance to heat and compression set, to a process for making such elastomeric blends, and to shaped or molded articles made from such blends.